Devices for stirring may be used in vessels for mixing liquids and/or granular material for instance in process industry. Devices for stirring may also be used for stirring contents of a vessel while the contents are heated or cooled or while a chemical or biological process takes place in the vessel. Generally a stirring agitator is arranged inside the vessel whereas a driving arrangement for rotating the stirring agitator is arranged outside the vessel.
The rotation may be transferred from the driving arrangement to the stirring agitator by means of a drive shaft extending through a wall of the vessel from outside the vessel into the vessel and the stirring agitator. The drive shaft must be sealed by means of a seal between the stationary vessel and the rotating drive shaft. There is a risk of such seals contaminating liquid or other material inside the vessel with seal material and there is a risk of leakage through the seal. After stirring and emptying of the vessel there is also a problem with cleaning such a seal from liquid or material that has been stirred in the vessel. In particular, such a seal may comprise cavities, in which liquid or material may collect and which are difficult to clean.
Therefore, contactless transfer of rotation from the driving arrangement outside the vessel to the stirring agitator inside the vessel may be preferred in industry where hygienic requirements are high, such as medical, food or biotech industry.
Contactless transfer of rotation to the stirring agitator has the advantage that there is no need for a rotating axle to extend through a wall of the vessel and may be achieved by means of a first set of magnets arranged in a drive shaft outside the vessel and a second set of magnets arranged in the stirring agitator inside the vessel. Such a stirring agitator is supported by one or more bearings inside the vessel. In particular a bearing subjected to axial forces, i.e. when the stirring agitator is arranged to rotate about a vertical axis, is prone to wear and may thus contaminate a liquid or material inside the vessel.
US2004/0047232 A1 discloses a system using a rotating mixing element where permanent magnets are used to provide a levitation-assist function to prevent the mixing element from contacting an adjacent vessel. The permanent magnets are ring magnets arranged at a distance from an axis of rotation.
U.S. Pat. No. 7,396,153 discloses a magnetically coupled liquid mixer. A first magnet array in a rotating drive hub affects a second magnet array in a driven portion such that there is a space, S, between the driven portion and a drive mount for supporting the driven portion in a vessel. The first and second magnet arrays are arranged in a ring at a distance from an axis of rotation. In some disclosed embodiments thrust forces on the driven portion make a first and a second thrust bearing come into contact with each other during operation, in which case the first and the second thrust bearings may be subjected to wear as mentioned above.
U.S. Pat. No. 5,393,142 discloses an impeller for stirring sterile liquids. The impeller is mounted by means of a bearing providing rotatable fixation with a seating surface in a container. The use of a bearing which is subjected to axial forces will mechanically affect the liquid to be stirred, which may not be desirable for certain liquids. Axial forces on the bearing may also entail that dust from the bearing due to mechanical wear ends up in the container.